Unsymmetric gain control device



June 10, 1941 J. l. HATHAwAY 2,244,695

UNSYMMETRIC GAIN 'CDNTROL DEVICE Filed sept. 2s, 195e lINP!! 7' LE I/.EL

IN1/Enron. JAR/ETT .HAT/:AWAY

BY' H6. ,W

A TTORNEY.

Patented June 10, 194i UNSYMMETBIC GAIN CONTROL DEVICE Jarrett L.Hathaway, Manhasset, N. Y., assignor to Radio Corporation of America, acorporation of Delaware `Application September 29, 1938, No. 232,273

This invention relates to gain control devices and has particularly todo with a system having a timing circuit for automatically governing thedynamic level range in audio frequency circuits.

My invention has particular utility in microphone circuits andamplifiers and may be associated with modulators for radio transmitters,or with public address systems, or sound recorders. Gain control isgenerally required in such systems and quite frequently it is necessaryto compress the volume range within narrower limits than those actuallyoccurring in the rendition of speech and musical programs.

'I'he gain control device herein disclosed is an improvement upon a gaincontrol device as covered in my copending application, U. S. Patent No.2,221,541 issued November 12, 1940.

Gain control devices as designed in accordance with the disclosure of myaforesaid copending application had the draw-back that they weresomewhat complicated and involved the use of a number of electrondischarge tubes and other equipment which, according to the instantapplication, have been found to be unnecessary. The results attained,however, in the improved arrangement appear to be at least equal tothose of the former arrangement.

The successful application of an automatic gain control device to theproblems of monitoring a broadcast program involves the use of timedelay circuits which will operate to rapidly attenuate a high levelaudio peak and afterwards to restore the normal gain in the circuit at agrad ual rate.

Accordingly, it is an object of my invention to provide a system ofautomatic audio gain control in which perceptible wave form distortionis avoided.

It is another object of my invention to provide a very simple system ofgain control apparatus which may be actuated solely in response tovvariations in an output component of audio frequency energy derived froma push-pull amplifier. It is another object of my invention to provide asystem of the class above indicated in which severe changes of accent,emphasis, or expression of speech and program are avoided.

It is a still further object of my invention to provide a means forautomatic gain control in which high level surges are instantly dealtwith by reducing the gain in the amplifier circuit so as not to overloadthe equipment following the control apparatus.

Other objects and advantages of my system will become apparent in thefollowing detailed description when read in View of the accompanyingdrawing, in which- Figure l shows diagrammatically a preferred circuitarrangement including volume control means for application to certainportions of an audio frequency amplifier system; and

Fig. 2 shows diagrammatically the operating characteristic of the gaincontrol device.

By way of review of the state of the art as represented, for example, inmy copending application aforementioned, it may be well to present thefollowing facts as a preliminary to the more detailed description of theinstant invention as will be hereinafter presented.

Up until quite recently it has been the practice to depend wholly uponmanual gain control in panies this gain reduction is insignificantbecause it takes place in so short a period of time as .005 second andthe human ear cannot recognize an effect lasting for so short a period.It is found desirable, however, following the application of a suddensuppression of a high level peak to restore the normal level ofamplification by slow degrees. The gain recovery is, therefore,accomplished without audible distortion. `The entire program is thuscompressed within suitable limits of volume range, considering theparticular sound reproducing equipment to be employed, and thetransmitter or other circuits involved.

Referring now to the drawing, I show a gain control circuitrinterposedbetween input terminals 5 and output terminals 6. The input terminalsare connected to the primary winding 'i of a transformer 8 whichpreferably has a split secondary, with winding 9. This winding feeds theinput signal to the control grids i I of two push-pull amplifier tubesl2. The tubes l2 are preferably of the pentode remote cut-off typehaving screen grids l5 and suppressor grids i6, the latter beingconnected to the cathodes l1. The anodes Il are connected to oppositeterminals of the primary winding I 9 of a coupling transformer 2l. Thistransformer has a secondary winding 2| having a mid-tap connectedthrough a resistor 22 to a tap 23 on a potentiometer 24. The terminalsof the secondary winding 2| are connected to the respective controlgrids 25 of two additional push-pull amplifier tubes 26 constituting asecond stage of amplification. The tubes 26 are likewise preferably ofthe pentode remote cut-oi type comprising each a cathode 28, theaforementioned control grid 25, a screen grid 29, a suppressor grid 36and an anode 3l. The anodes 3l of the tubes are connected respectivelyto the terminals of a transformer primary winding 32 having a centertap. This primary 32 is appropriate to an out- Y put transformer 33which also has a secondary 34 connected to any suitable utilizationcircuit. In order to adjust the gain of an auxiliary ampliiier 4| to asatisfactory normal level, and also in order to improve the fidelity ofthe push-pull amplifier 26, two resistors 35 are placed in shuntrespectively with the two paths of the transformer primary 32.

A source of direct current potential 36 is applied across thepotentiometer 24, the negative terminal of which is preferably grounded.Leading directly off from the positive terminal of the source 36 is aconductor 36 which connects with the mid-tap on the transformer primarywinding |9 and also to the mid-tap of the transformer primary winding32. Anode potential is thus supplied to the push-pull tubes l2 and 26.All of the cathodes of these tubes are connected to a tap 3l on thepotentiometer 24, ythis tap being .positioned to receive a slightlypositive potential relative to ground for biasing the tubes.

Suitable positive potentials for the screen grids l and 29 are obtainedfrom a tap 36| on the potentiometer 24.

In order to apply a normal rio-signal bias potential to the controlgrids and 25, I preferably employ a biasing circuit which may be tracedfrom ground through a high resistor 22 having a value of several megohmsand through the secondary windings 2| to grids 25, also through aresistor 40 and the secondary windings 9 to the grids of the tubes l2respectively. This biasing circuit is, however, subject to gain controlaction by means of a double diode rectifier amplier tube 4| incombination with a transformer 42 having a primary winding 43 and asecondary Winding 44, the latter having a center tap connected to thebiasing circuit above traced. ,The terminals of the secondary winding 44are connected respectively to each of the two diode rectifier anodes 45within the tube 4|. This tube also comprises a cathode 46 which may beof the indirectly heated type, an amplifier control grid 41 and anamplifier anode 48. The cathode 46 is connected to the potentiometer 24at a tap 49 which is positioned to receive a more positive potentialthan that of the tap 23. The anode 48 is connected to the positiveterminal of the source 36, this connection being made through primarywinding 43. The control grid 41 is negatively biased as by means of gridresistor 50, connected to potentiometer tap 23.

Control grid 41 receives audio voltage from the output of tubes 26 byway of a tap on potentiometer 35. The direct current path from the platesupply of tubes 26 is blocked by condenser 5I. The amplifier section oftube 4| feeds into the primary 43 of transformer 42. The center tappedsecondary of this transformer supplies audio voltage to the dioderectier sections of tube 4l. The resultant rectified output D. C.voltage thus appears across the resistor 22,y the pulsations beingsmoothed out by condenser 52. An initial bias on tube 4| diode rectiiierelements is created by the position of tap v49 on the potentiometer 24.Rectification thus can not set in until the audio voltage peaks at thediode plates exceed this initial bias The purpose of this arrangement isto prevent rectification and the consequent control action which wouldotherwise occur at abnormally low input signal levels. Because of theinitial bias, rectification starts at a definite input level, andfurther increase of inputlevel causes an extremely rapid bias increase.The relative rate of bias increase with input level increase is a directfunction of the initial bias. This makes possible the sharp knee on thecontrol curve shown in Fig. 2.

The rectified biasing voltage applied to grids 25 of tubes 26 increasesvery suddenly when the signal applied to the input terminals reaches asufficient level. The time constant of the circuit supplying the bias isdetermined by the value of the charging resistance in relation to thecapacitance of the condenser 52. The charging resistance is the sum ofthe diode resistance and the section of transformer secondary winding 44which supplies the audio peak. The discharge time, following an audiopeak is determined by the time constant of the condenser 52 and thedischarge resistor 22. This time constant is many thimes greater thanthe charging time constant. The gain, therefore, is restored to normalat a relatively slow rate.

The full-line curve shown on Fig. 2 indicates the control actionafforded by the remote cut-oli tubes 26 only. This full-line curveillustrates the possibility of compressing the output volume range abovethe amplitude level of the knee in the characteristic to within about 2decibels while the input level varies over a range of 20 decibels. Ifthe control action were to be removed, then the output level would tendto vary in direct proportion to the input level, as shown by thelight-line extension of the steep slope above the knee.

From the diagram it will be seen that the tubes l2 may be subjected tovolume control simultaneously with the control of tubes 26, if desired.For this purpose a switch 53 is provided, and when set in the positionshown the grid return circuit for the tubes l2 is extended from thegrids 'through transformer windings 9 and l0, thence through resistor 40and switch 53 to the mid-tap of transformer secondary 44 from which arectified control voltage is derived. When automatic volume control isnot to be applied to tubes l2, then the switch 53 is set so as to groundthe grid return circuit, or, if desired, to impress thereon any suitableD. C. bias, such as may be obtained from a given tap on thepotentiometer 24. In this condition the tubes |2 act merely as ampliers.

I have found it advantageous, however, to control the bias on the gridsof successive tube stages I2' and 26 so that the rate of bias increasein these two stages is radically diierent. Accordingly, the biaspotential is applied to tubes I2 at a relatively slow rate by virtue ofthe time required to charge condenser 54 through resistor 40. Also thedischarge time of condenser 54 is somewhat slower than that of condenser52. Thus both in gain reduction and gain recovery the tubes l2 are moreslowly controlled. This action alleviates part of the control requiredin the unsymmetrically timed circuits associated with tubes 26. However,the rapid gain control action afforded by tubes 26 in cutting downsudden high peaks is always available. The dot-and-.dash line in Fig. 2shows the resultant control characteristic curve which is obtained whenthe switch 5l is set for controlling the gain in tubes I2 along withtubes 26, and when the input is a steady tone. Here the output volumerange above the amplitude level of the knee in the characteristic curvemay be compressed to within half a decibel for an input range of 40decibels.

If desired, resistance coupling may be employed between the twopush-pull stages in place of transformer 20, provided'constants are sochosen as to permit grids to reduce gain at a sumciently rapid rate forgood control action.

More than two successively controlled stages may be employed, using thesame rectiied voltage to affect the gain control. Time constants may bethe same in all, or different, as described above. In the case ofdifferently timed circuits the slower timed circuits may be locatedeither ahead or behind the circuit which acts to cut gain rapidly.

The diode rectiiier in tube ll'may be half wave instead of full wave, asshown. If half wave rectiiication is used, one of the diode plates isdisconnected from the transformer terminal of secondary M and connectedto the opposite end of the secondary Such an arrangement is founddesirable in certain applications.

It is within the scope of my invention to apply the teachings thereof toother embodiments than those herein shown and described. The inventionitself, therefore, is limited only in accordance with the claims.

I claim:

1. A gain control system applicable to a twostage push-pull dischargetube amplifier, said system comprising a. discharge tube having acathode, a control grid, an anode, and an anode rectier device adjacentthe cathode; a source of direct current operating and biasing potentialscommon to said amplier and said tube, a grid biasing circuit connectedto the control grids of said push-pull ampliier and to said directcurrent source, said circuit being branched to the grids of successivestages, means including a time constant device in the biasing circuitbranch of the preceding stage for retarding the control action of thatstage in relation to the control action on the subsequent stage, atransformer having a primary winding in the output circuit of saiddischarge tube and a secondary winding the terminals of which areconnected in circuit between the cathode and the anode rectifierdevice'of said tube, a connection from said secondary winding to saidbiasing circuit; and a time constant device included in said biasingcircuit between the rectiiier device and the cathode of said dischargetube, said time constant device being characterized in that it providesrapid gain reduction and slow gain recovery in said amplifier only inresponse to audio wave peaks above a given amplitude level as deliveredby said ampliiier.

2. In a gain control system applicable to a twostage push-pull dischargetube amplifier having the usual electrodes including gain controlelectrodes, a. discharge tube having an ampliner section and a doublediode rectier section, means operative to control the ampliier sectionof said tube in response to audio wave peaks derived from the output ofsaid push-pull ampliier, a transformer having its primary in circuitbetween output electrodes of said amplier section, said transformerhaving a center-tapped secondary the mid-terminal of which is negativelybiased ,with respect to the cathode of said discharge tube, the outerterminals of said secondary being connected respectively to the anod ofsaid double diode reciler section, and meansresponsiveonlytothoseotmidaudiowave peakswhich surpas a given amplitudekavel for enabling said dischargetubeincooperaonwithsaidtransformer toapply a rapid gain reduction control voltage and a relatively slow gainrecovery control voltage to said gain control electrodes of saidpush-pull ampliiier, and means including a time constant device incircuit with the gain control electrodofthcrststageinsaidampliierfordelayingthecontrolactiontbereoninreierence to the control action appliedto the subsequent stage.

3. In combination, a discharge tube provided with a grid-controlledampliier section and a diode rectiier section. a push-pull dischargetube amplifier system having an output circuit the voltage in which isavailable for control of theampliiersectioninsaiddischargetube,atransformer having a primary windingconnected in circuitwiththespacecurrentpathofsaiddischarge tubeampliiier section and a secondarywindingtheterminalsofwhichareincircuitwiththespacecurrentpathofsaiddischargetube diode rectiier section, anautomatic volume control circuit for said push-pull ampliier system, thelast mentioned circuit being connected to a mid-tap on said secondarywinding, and a time constant device included in said automatic volumecontrol circuit ami characterized in that it provides a gain reductionin said push-pull amplifier system which is substantially instantlyresponsive to audio wave peaks above a given amplitude level in theoutput of said push-pull amplifier system, and a much retarded gainrecovery following such gain reduction.

LThecombinationsetforthinclaimland including branches in said automaticvolume control circuit for controlling two stages in said push-pullampliler system, and means `including a separate time constant device inthe branch to one of said stages whereby both the gain reduction and thegain recovery in said stage are delayed with respect to thecorresponding gain reduction and recovery of the other stage.

5. The combination set forth in claim 3 and further characterized inthat said time constant device includes a capacitor shunting a highlyresistive impedance in circuit between said automatic volume controlcircuit and ground.

6. An automatic volume control circuit arrangement for controlling apush-pull amplifier, comprising a grid biasing circuit appropriate tesaid `amplifier, means for amplifying an output component of saidamplifier, a transformer having a primary fed with energy derived fromsaid amplifying means, a diode rectiier having anodesconnectedtotheterminalsofasecondaryonsaid transformer and a cathode incircuit with a midtap on said secondary, a source of operating andbiasing potentials common to said push-pull amplifier, its grid biasingcircuit and said amplifying means; said grid biasing circuit includingsaid secondary mid-tap and running thence through a resistor to a pointof suitable bias potential on said source, and a capacitor in shunt withsaid ristor; the combination of said resistor and capacitor constitutinga time constant means such that it renders said grid biasing circuitoperative to suddenly reduce the gain of said push-pull amplier afterthe output component thereof has reached a given amplitude level,andtoslowlyincreasemegaininresponsetoa subsequent reduction of saidoutput component to a valueg below said given amplitude level.

7. An automatic volume control circuit arrangement for controlling atwo-stage push-pull ampliiler, comprising, in combination, an electronicamplifier under control of output energy from said push-pull amplifier',a transformer having a primary winding inuenced by output energy fromsaid electronic amplifier `and a secondary winding, a mid-tap on saidsecondary winding, said midtap being capacitively coupled directly toground and being directly connected to the input side of said push-pullampliiler, an electronic diode rectifier having direct anodalconnections with the terminals of said secondary winding, and a bias- 15mimosas ing source in circuit between the cathode-side oi said rectiierand the mid-tap of said secondary winding, and means including two timeconstant devices in said volume control circuit arrangement, one of saiddevices being operative to provide a substantially instant gainreduction in said push-pull amplifier in response to wave peaks of saidoutput energy only above a predetermined amplitude level, and to providea relatively slow gain recovery thereafter, while the other of said timeconstant devices is operative to retard the control action on oneampller stage with respect to the other.

JARRETT L. HATHAWAY.

